Charge coupled devices, hereafter referred to as CCDs and bucket brigade devices (BBDs) are both charge transfer devices (CTD) capable of handling analog information. Continuous recirculation is required for long term memories. As is known, upon each transfer of a given charge packet, a small amount of charge is left behind. The amount depends upon the charge transfer efficiency of each device. This action progressively diminishes the amplitude of the original charge packet and erroneously increases the charge in subsequent packets. Therefore, since charge transfer efficiency is not perfect, analog information becomes smeared out along the device. In addition to dispersion distortion, a constant accumulation of dark current will eventually saturate all of the packets. Thus, simple recirculation of analog information in an analog CCD shift register normally fails because of the amplitude and phase distortion due to transfer inefficiency and the accumulation of dark current.
Dispersion effects can be compensated by use of a transversal filter. Losses and the distortion of information in the shift register may be compensated by use of a series transversal filter. The transfer function H(eq) for the equalization may be expressed in terms of a gain factor and a phase factor proportional to the charge transfer inefficiency: ##EQU1## WHERE .beta.IS A GAIN FACTOR,
.xi.is transfer inefficiency, PA1 n is the number of transfers in the CCD, PA1 fc is the clock frequency, and PA1 f is the frequency of the analog signal sampled and applied to the CCD input.
As to dark current, the charge in each packet contributed thereby is temperature dependent, doubling in magnitude in typical devices for each 10.degree.C change. Dark current may be compensated by a shift in the D.C. level of the signal. However, both the dispersion effect and the dark current effect cannot be compensated simply by gain.